1. Field of the Invention
The present invention relates to transmission antenna diversity. More specifically, the invention relates to a closed-loop transmission antenna diversity method, and a base station apparatus and a mobile station apparatus therefor, in a next generation mobile telecommunications system.
2. Description of the Related Art
A third generation mobile telecommunications system has a standard for higher-speed transmission of data than the previous, second generation mobile telecommunications system. The second generation mobile telecommunications system is represented by personal communication systems (PCS).
As a wireless connection specification, the W-CDMA method, an asynchronous method, is being standardized in Europe and Japan. Concurrently, the IS-2000 CDMA method, a synchronous method, is being standardized in North America. A mobile telecommunications system is constructed in such way that a plurality of mobile stations communicate with each other through one base station.
In a mobile telecommunications system, fading can be a significant problem. Fading reduces the amplitude of a received signal by a few decibels (dB) to tens of decibels. In order to transmit high-speed data, fading must be sufficiently overcome. A variety of diversity technologies are used to overcome fading. The CDMA method adopts a rake, which receives signals in diversity reception using the delay spread of a channel. The rake is a multipath diversity technology. A deficiency of this technology is that it does not operate when the delay spread is small. Time diversity technology, which uses interleaving and coding, is used in a Doppler spread channel. It is difficult to use this time diversity technology in a low-speed Doppler channel.
A space diversity method is used in order to overcome fading in an indoor channel, which has small delay spread, and in a pedestrian channel, which is a low-speed Doppler channel. The space diversity method uses two or more antennas. When a signal received in one antenna is attenuated by fading, the other antenna is used for receiving the signal. Space method antenna diversity is divided into receiving antenna diversity, which uses receiving antennas, and transmitting antenna diversity, which uses transmitting antennas. Since it is difficult to install receiving antenna diversity in a terminal from the aspects of space and cost, use of transmitting antenna diversity in a base station is recommended.
In addition, transmitting antenna diversity is divided into closed-loop transmission diversity, in which uplink channel information is fed back from a terminal, and open-loop transmission diversity, which has no feedback operation. The closed-loop transmission diversity has twice the gain of open-loop transmission diversity in the aspect of signal-to-noise and interference ratio (SNIR). However, the closed-loop transmission diversity, which operates by feeding channel information back, has a shortcoming in that it is greatly affected by the feedback cycle. When a feedback cycle is long, performance is lowered because a channel changes before feedback information arrives at a terminal. When a lot of information per unit time is fed back in order to trace a rapidly-changing channel, the capacity for uplinking is lowered. Therefore, when a channel changes rapidly due to a high Doppler frequency, it is more helpful to performance improvement to quickly transmit minimum necessary information than to transmit detailed information on a channel.
Additionally, according to a diversity combination mode, transmitting antenna diversity is divided into a maximal ratio combination (MRC) mode, an equal gain combination (EGC) mode, and a selective combination (SC) mode. The EGC mode is a little lower in performance compared to the MRC mode, but is widely used because the peak-to-average ratio (PAR) of the EGC mode is lower. The MRC mode, of which the PAR is higher, raises the cost of a transmission power amplifier in a CDMA base station.
FIG. 1 is a block diagram showing the organization of a base station using a general transmission antenna diversity method operating in an equal gain mode for coherent combination, as known in the prior art.
Referring now to FIG. 1, an i-th mobile station obtains the phase difference between antennas by measuring information on two separate channels transmitted by two transmitting antennas 100 and 110 of a base station, and feeds the phase difference back to the base station. The base station decodes the phase difference, which is feedback information, through a feedback information decoder 120, and phase-compensates the decoded result through a phase adjuster 130. A data signal, which is generated in a transmission signal generator 140, is transmitted together with a first pilot signal through the first antenna 100, and the same data signal weighted with the phase-compensated phase difference, is transmitted together with a second pilot signal through the second antenna 110. The apparatus of FIG. 1 also includes a multiplier 150 and a first and a second adder 160 and 170, respectively.
Two United States patents, U.S. Pat. No. 5,634,199, entitled “Method of Subspace Beamforming Using Adaptive Transmitting Antennas with Feedback” and issued to Derek Gerlach, Columbus, Ohio; Arogyaswami Paulraj, Stanford, Calif.; and Gregory G. Raleigh, El Grannanda, Calif., on May 27, 1997 and U.S. Pat. No. 5,471,647, entitled “Method for Minimizing Cross-talk in Adaptive Transmission Antennas” and issued to Derek Gerlach, Los Altos Hills, Calif.; and Arogyaswami Paulraj, Stanford, Calif. relate to transmitting antenna diversity methods. The former patent is assigned to Stanford University, CA, U.S.A., and the latter patent is assigned to The Leland Stanford Junior University, CA, U.S.A. Both patents use transmit-diversity in a feedback mode.
These patents propose channel measuring and feedback methods using a perturbation algorithm and a gain matrix. These methods are not optimal methods for efficiently processing channel information.
In the meantime, another method has been proposed in which channel information is efficiently quantized and fed back, as also depicted in FIG. 1. Specifically, when transmission diversity, which operates in an equal gain mode, is used, the phase difference with a base antenna is quantized by two-bit information {0, π/2, π, −π/2} or one-bit information {0, π}.
The equal gain mode is a diversity combination method in which when destructive interference occurs due to the phase difference of two transmitting antenna channels, phases of diversity antennas are compensated in advance with the phase of a base antenna so that a signal received through each antenna channel has the co-phase. The phase difference with a base antenna is obtained by subtracting the phase of a first antenna channel, that is, a base antenna channel, from the phase of a k-th antenna channel.
The two-bit information equal gain mode optimally quantized channels, but still more reduction in bits is needed in order to satisfy rapidly changing channels. Quantizing in one-bit causes the loss of a lot of information, and therefore, is inferior to the two-bit mode, except for high-speed transmission. A method for feeding back two-bit information with a faster cycle has been proposed, but this method has a shortcoming of greatly reduced uplink capacity.